What would I like for the next DS? hmm...
How about a Star Fox or F-Zero game?
A nice sized harddrive, SD card port, better wifi and an online gamer profile that isn't cumbersome.
Skype would be nice, from playing a lot of WoW recently has shown me how awesome the service is.
Because nintendo has the unwritten "only one installment per system" rule, especially when it comes to multiplayer centric affairs, they should really look into DLC to constantly give their game legs.
Also, I want to be met in the middle with achievements. While I think they do extend a game's shelf life a bit, I do think they're a lazy way to do so. How about making the games have less achievements but they unlock something gamewise like an extra skin or new kart, but still have a composite overall score that can be viewed by other gamers. Hell, just make our score coins or stars or something nintendo-y.
Also, open pricing on the games/apps. No reason why I should pay 2 bucks for a animal crossing skinned clock.
Mii's are awesome. Please keep/use those.
Last but most important... HAVE BUTTONS!